Orthogonal frequency division multiplexing (OFDM) is a common approach to implement a multi-carrier communication system that uses a very large bandwidth over a frequency-selective communication medium. OFDM systems typically use multiple carriers at different frequencies that experience different fades, for example, due to different channel attenuations and different phase rotations. Link adaptation deals with tailoring the amount of data on a carrier based on the physical characteristics of the communication medium.
Link adaptation including, for example, bit-loading can be performed at various granularities. In some cases, the code rate and/or modulation size is adapted for different OFDM symbols and all carriers in a given OFDM symbol use substantially the same code rate and modulation size. In other cases, the carriers in each OFDM symbol can have the same code rate but different modulation size. Alternatively, multiple code rates can be achieved on each OFDM symbol, for example, by spreading the encoded bits on carriers associated with the same code rate across multiple OFDM symbols. Such an approach may increase the latency of the system, for example, if all of the multiple symbols need to be received for decoding. Multiple code rates can also be achieved by assigning small encoded blocks to carriers associated with the same code rate in a single OFDM symbol. This approach reduces the latency, but produces lower coding gains because of the reduced block size.